Field of the Invention
The present invention relates to the field of the coating of fibers by liquid means, and deals in particular, but not exclusively, with a device and with a method for obtaining a thick and uniform metal coating on a fiber, in industrial production conditions.
Description of the Related Art
In the aeronautics field, in particular, it is a constant objective to optimize the strength of the parts for minimum mass and size. Thus some parts may now comprise an insert of composite material having a metal matrix. Such a composite material comprises a metal alloy matrix, for example of titanium alloy, in which fibers extend, for example ceramic fibers of silicon carbide. Such fibers have a much higher tensile strength than that of titanium (typically 4000 MPa compared to 1000 MPa) and a stiffness that is typically three times greater. Hence these fibers can absorb the forces, the metal alloy matrix transferring the loads between the fibers, acting as a binder with the rest of the part, and protecting and separating the fibers, which must not be in contact with one another. Furthermore, ceramic fibers are strong, but brittle, and must necessarily be protected by metal.
These composite materials can be used in the production of disks, shafts, cylinder bodies, casings, spacers, as reinforcements for monolithic parts such as blades, etc. They can also find an application in other fields in which a volume force field is applied to a part, for example a pressure vessel such as a pressurized fluid tank.
In order to obtain such an insert of composite material, threads called “coated threads” are first formed, comprising an armature formed from a ceramic fiber, coated with a metal sheath. The metal coating imparts higher stiffness to the thread but also better toughness, which is useful for its handling. The manufacture of threads of composite material, or coated threads, can be carried out in various ways, for example by metal vapor deposition on the fiber under an electric field, by electrophoresis from metal powder, or by dip-coating the fibers in a liquid metal bath.
A method for coating fibers by dipping in a molten metal bath is described in patent application EP 1995342 or patent EP 093 1846, filed in the name of the Applicant. Patent EP 093 1846 describes a method for metal coating of fibers by liquid means, said method being more particularly, but not exclusively, intended for coating fibers with metals and metal alloys having a high melting point. In this method, the liquid metal is kept in levitation in a suitable crucible, at an appropriate temperature, in order, at least partially, to prevent contact with the walls thereof. The levitation is obtained by electromagnetic means surrounding the crucible. The ceramic fiber, kept taut by gripping means, is drawn through the metal bath. A device serves at will to make the fiber pass through the crucible so that it takes up metal while passing through the metal bath, or to make it circumvent the bath. The speed of passage of the fiber in the metal bath is set according the desired metal thickness on the fiber. In view of the viscoelastic properties of molten titanium, the speed yielding the highest titanium thickness is about 3 m/s.
One of the problems that arises in these high-speed ceramic fiber coating processes is associated with the need to cool the coated thread rapidly after its passage through the liquid metal ball. In the prior art, the cooling is carried out by a column through which the thread passes and which has a large number of cooling nozzles which eject air toward the thread when it emerges from the metal ball. It is known that the thread must be cooled to a temperature of about 500° C. to obtain complete solidification. In the case of a 140 micron thick silicon carbide fiber which emerges from a liquid titanium bath at 1700° C. and which travels at a speed of 3 m/s, it is typically necessary to have a column that is one meter high to obtain the desired solidification while ejecting air under a pressure of 2 bar.
The drawbacks associated with this method are, first, a large size of the installations, but also, and above all, a mediocre quality of the thread, because of the appearance of instabilities on the diameter of the sheathed thread which imparts a wavy appearance to the metal coating. These longitudinal instabilities are all the more marked if the relative thickness of the sheath compared to the fiber is high. One of the consequences is a risk of breakage of the coated thread, at the constrictions of its diameter, when it is wound on storage drums after coating.
To avoid this, the person skilled in the art is currently limited to relatively high fiber contents, the fiber content being defined as the ratio of the diameter of the initial fiber to the diameter of the coated thread. In fact, it is desirable to be able to produce threads having a high relative sheath thickness, that is to say, a low fiber content, and at all events, lower than the 35% stated in patent application EP 093 1846. The production of low-content threads faces the problem of longitudinal instability of sheath thickness, which is caused by the velocity shear in the meniscus of the liquid sheath that is entrained by the fiber, and by the natural tendency of a liquid to take a position in which its surface tension is minimized. Since the ideal shape for a liquid free to be deformed is a sphere, the sheath tends to be deformed to assume a shape similar to that of a string of droplets carried by the fiber. This longitudinal instability of the sheath is all the more pronounced if the fiber content is low and the time taken for cooling is long.